1. Field of the Invention
The present invention relates generally to methods and materials by which fluorescent display screens are manufactured. More particularly, the present invention relates to methods for forming low energy electron excited fluorescent phosphor compositions upon fluorescent display screen substrates, which fluorescent phosphor compositions provide fluorescent display screens having decreased threshold voltages.
2. Description of Related Art
The use of phosphors as fluorescent elements in the production of low-energy (ie: low velocity) electron excited fluorescent screens has been known for many years. The traditionally phosphor employed in fabricating low-energy electron excited fluorescent screens is the zinc activated zinc oxide (ZnO:Zn) phosphor. Although many other phosphors traditionally have been known, the zinc activated zinc oxide (ZnO:Zn) phosphor was unique among traditional phosphors in its ability to fluoresce under low-energy electron excitation conditions, typically at accelerating potentials of less than 100 volts. While many other phosphors fluoresced, they did so only under substantially higher electron excitation conditions, typically in the range of kilo-volts. The low-energy electron excited zinc activated zinc oxide (ZnO:Zn) phosphor fluoresced to produce a green-white image, and the phosphor was used as the active fluorescing element in various types of low-energy electron excited fluorescent display screens for electronic calculators and measuring devices.
As demand for low-energy electron excited fluorescent screens of various color types and increased color purity developed, so also were developed different classes of fluorescent phosphor compositions which met those demands. A large group of such compositions is disclosed by Hase et al., in U.S. Pat. No. 4,081,398 and U.S. Pat. No. 4,208,613. The disclosed compositions include traditional high-energy electron excited fluorescing phosphors mechanically mixed with indium oxide. Although not entirely well understood, it is felt that the improved fluorescence efficiencies of the phosphor/indium oxide fluorescent phosphor compositions derives from the increased electrical conductivity of the phosphor/indium oxide fluorescent phosphor compositions as a whole. With increased electrical conductivity of the fluorescent phosphor compositions, charge up of the phosphors within those compositions does not occur on occasion of low-energy electron excitation of those phosphors.
The low-energy electron excited fluorescent phosphor compositions disclosed by Hase et al. are employed in producing low-energy electron excited fluorescent screens of high color purity in color hues including red, blue and green. Methods through which fluorescent phosphor screens are prepared through these fluorescent phosphor compositions include sedimentation of an aqueous suspension of the phosphor/indium oxide fluorescent phosphor composition onto a fluorescent screen substrate, with subsequent thermal drying.
Subsequent to the Hase et al. disclosure, various additional fluorescent phosphor compositions exhibiting unique or enhanced properties have been disclosed. Methods through which these additional compositions have been formed include: (1) doping of traditional phosphors with metals such as aluminum, copper and zinc; (2) additional mixing of traditional phosphors with conductive oxides such as indium oxide and tungsten oxide; and (3) thin-film processing of traditional phosphors onto surfaces of electrically active particles. Some compositions are susceptible to low-energy electron excitation, other compositions are not. For example, Bryan et al., in U.S. Pat. No. 4,992,205 disclose indium doped and titanium activated fluorescent phosphor compositions used in long-wavelength emitting intensifying screens for x-ray exposure applications. In addition, Reilly et al., in U.S. Pat. No. 5,009,808 describe a method for producing an electro-luminescent zinc sulfide phosphor activated with manganese, chloride and copper. Further, Takahashi et al., in U.S. Pat. No. 5,032,316 describe a uniform high luminescence stability zinc oxide activated zinc-cadmium sulfide fluorescent phosphor composition containing alumina.
Still further, Yoneshima et at., in U.S. Pat. No. 5,055,227 disclose high luminescence low energy electron excited fluorescent phosphor compositions formed through mixture of traditional phosphors with indium oxide of specific crystallinity levels. Yet further, Inaho et al., in U.S. Pat. No. 5,102,579 describe a novel method for decomposing a metal sulfide and thereby forming a sulfurizing atmosphere within which are formed sulfide phosphors. Finally, Karam, et al., in U.S. Pat. Nos. 5,273,774 and 5,309,371 describe a zinc sulfide electro-luminescent phosphor of high luminescent intensity formed through a thin-film process.
In addition to the above recited art which is primarily directed towards the chemical compositions of fluorescent phosphor compositions, there also exists additional art relating to methods by which fluorescent phosphor compositions may be coated upon suitable substrates to form fluorescent screens. Typical coating methods include sedimentation, centrifugation and electrophoresis. For reasons of manufacturing efficiency and reproducibility, electrophoretic methods are often preferred.
Electrophoresis and related electroplating methods are disclosed by Hennicke et al., in U.S. Pat. No. 4,246,086 and Niksa et al., in U.S. Pat. No. 5,017,275. Electrophoretic deposition of luminescent materials has been known for several years, as disclosed by Cerulli in U.S. Pat. No. 2,851,408.
Most pertinent to the present invention, however, are the disclosure of Shane et at., "Electrophoretic Phosphor Deposition for CRTs," SID 93 Digest, pp. 542-45 and the disclosure of Siracuse et al., "The Adhesive Agent in Cataphoretically Coated Phosphor Screens," 137 J. Electrochem. Soc., No. 1, 346-48 (1990). Both of these disclosures address the chemical mechanisms through which alcoholic solutions of magnesium nitrate into which is suspended a fluorescent phosphor composition form upon suitable substrates a fluorescent screen derived from the fluorescent phosphor cemented upon the substrate within a magnesium hydroxide binder.
Desirable in the art are methods which simultaneously exploit a knowledge of the materials through which are formed low-energy electron excited fluorescent phosphor compositions and a knowledge of the methods by which are formed fluorescent screens upon fluorescent screen substrates through coating those fluorescent screen substrates with low-energy electron excited fluorescent phosphor compositions. Through a knowledge of both the low-energy electron excited fluorescent phosphor compositions and methods by which those fluorescent phosphor compositions may be coated to form low-energy electron excited fluorescent screens, there may be formed more efficient low-energy electron excited fluorescent screens with reduced threshold voltages. The foregoing represents the object towards which the present invention is directed.